This proposal aims to elucidate the functional behavior of the microcirculation in sickle cell disease (SCD) by the two pronged approach of: (1 Studying the biophysical behavior of human HbSS cells infused as bolus injections into the microvasculature of laboratory animals and (2) studying capillary blood flow in the skin (nailfold of the finger) of the sickle cell disease cell patient. The animal studies aim to examine the relationship between hemoglobin deoxygenation and transit time of red blood cells (rbcs) throughout successive microvascular divisions to delineate conditions which give rise to HbSS hemoglobin polymer formation and its effect on the convective flux of oxygen in the microcirculation. Preliminary studies suggest that reductions in tissue oxygen tensions below 20 mmHg serve to increase the transit time (TT) of HbSS rbcs, thus supporting the "vicious cycle" of further flow degradation and red cell entrapment. This process will be studied by bolus infusions of HbSS rbcs into the cremaster muscle (rat and mouse) to assess the role of HbSS rbc deformability and red cell density as determinants of the onset of rbc sequestration in deoxygenated tissues. Red cell transit time, TTrbc will be measured by application of techniques of indicator dilution of fluorescently labelled rbcs as a bolus (0.05 ml) of HbSS cells traverses the microvascular network while viewed under fluorescence microscopy. The introduction of a larger bolus (.1 to .2 ml) will facilitate the measurement of regional resistance between functionally paired arterioles and venules, and the resistance to flow in single unbranched microvessels by the measurement of intravascular pressures and pressure gradients (dual servo-null technique) and red cell velocities (two-slit photometric method). Spectrophotometric determinations of the percent hemoglobin oxygen saturation and microvessel hematocrit (three wavelength method) will facilitate quantitation of the resistance to oxygen transport from blood to tissue and the extraction of oxygen from the human blood samples. Studies aimed at contrasting the effects of rbc density and mean cell volume will also be conducted by the study of blood from HbSC and HbSS alpha-thalassemia patients. Measurements of transit time and regional resistance of HbSS blood will also be conducted in the pial circulation (rat) to seek correlates with the occurrence of stroke and the cerebrovascular hyperemia frequently found in the SCD patient. Parallel studies of skin capillary blood flow in SCD patients will examine the rheological conditions leading to vaso-occlusive crisis. Video recordings of capillary blood flow in the resting state, during an induced low flow state (compression of digital arteries by a pressure cuff) and during recovery from the low flow state (post occlusive reactive hyperemia) will be made under crisis and crisis-free conditions to seek correlates with the onset of crisis in light of rbc adhesion to the endothelium, the state of leukocytosis and the aggregability of blood as factors which affect recovery from ischemic episodes. It is anticipated that the proposed studies will provide new and useful information to guide the clinical management of SCD.